This invention relates generally to a toner/carrier mix for developers in electrophotography, and more particularly to an improved carrier utilized in conjunction with a particular toner size classification.
In electrophotography, a photoconductor is charged and then exposed imagewise to light. In the area of the photoconductor exposed to light, the charge dissipates or decays while the dark areas retain the electrostatic charge.
The difference in the charge levels between the areas exposed to light and the dark areas produces electrical fields therebetween. Thereafter, the resultant latent electrostatic image on the photoconductor is developed by depositing small colored particles, which are known as toner particles, having a charge so as to be directed by the electrical fields to the image areas of the photoconductor to develop the electrostatic image.
A number of means are known for developing the latent electrostatic image by the application of the toner particles. One of these is known as cascade development and is described in U.S. Pat. No. 2,638,552 to Wise. Another means is known as the magnetic brush process. This method is described in U.S. Pat. No. 2,874,063 to Greig.
In each of the cascade and magnetic brush development processes, a two component developer material is utilized. The developer material comprises a mixture of small toner particles and relatively large carrier particles. The toner particles are held on the surfaces of the relatively large carrier particles by, among other factors, electrostatic forces which develop from the contact between the toner and carrier particles producing triboelectric charging of the toner and the carrier to opposite polarities. When the developer material is moved into contact with the latent electrostatic image of the photoconductor, the dislodged toner particles are attracted to the latent image.
The toner and carrier particles of the developer material are specially made and processed so that the toner obtains the correct charge polarity and magnitude of charge to ensure that the toner particles are preferentially attracted to the desired image areas of the photoconductor. The toner particles are then transferred electrostatically to the desired copy sheet, after which the transferred image of toner particles is fused by some combination of heat, pressure or solvent heat and/or pressure to produce the final product of a fused copy of the desired image.
One of the problems encountered is to provide the best possible quality of a final image on the copy sheet. This is generally referred to as copy quality. Copy quality includes such things as image clarity, i.e., clear delineation of lines; uniform darkness of the image areas; background quality, i.e., grayness or lack of it in the background areas; and other somewhat intangible features that go toward making a good "quality" copy.
Other factors that merit consideration in the developing process vis-a-vis toner is the overall utilization of toner per copy. Of course from an economic point of view the less toner used per any given image the better. Also in a system in which nontransferred toner is cleaned from the air by use of a filter, it is important to minimize the amount of nontransferred toner to thereby extend the life of the filter.
Further, when heat fusing is used it is desirable to provide an image that will need the least possible amount of heat to fuse the image. This is important not only from an energy point of view, but also with more rapid heat transfer by the toner, the fusing time or temperature can be reduced.
Said application Ser. No. 848,173 discloses a toner classification which to a large extent overcomes these problems. However, there are additional problems encountered when utilizing this improved toner with conventional carrier beads having polytetrafluoroethylene coating thereon. One particular problem encountered is what is known as fluorocarbon contamination. This is characterized by the transfer of the polytetrafluoroethylene from the carrier coating onto the surface of the toner. Such transfer or smearing causes a detrimental alteration of the triboelectric properties of the surface of the toner particles with a resultant deterioration in the performance of the toner.
Additionally, the pure polytetrafluoroethylene coating on the carrier has a slow, but still rather significant wear rate. Hence, while the polytetrafluoroethylene coated carrier is quite long lasting with respect to other conventional coatings, nevertheless it does wear off after several hundred thousand copies have been made.
Thus there is a need for a carrier coating for use in conjunction with the improved toner that reduces fluorocarbon contamination and is extremely long lasting.